LEARNING OBJECTIVES
By the end of this chapter, you should be able to:
- trace energy through food webs
- interpret ecological pyramids
- explain trophic losses
- describe carbon and nitrogen cycles
- interpret population growth
THE BIG IDEA
How energy, nutrients and populations connect ecosystems.
This chapter builds each process from its structures, inputs and outputs. Follow the connections rather than memorising isolated facts, then use precise biological terms when explaining cause and effect.
SECTION 01
Food webs and energy
Sunlight is the main energy input. Producers make nutrients; consumers feed on organisms; decomposers feed on dead and waste material.
DETAILED EXPLANATION
- Food-chain arrows show energy transfer from food to feeder.
- Consumers occupy primary to quaternary trophic levels; herbivores eat plants and carnivores eat animals.
- Food webs reveal effects of overharvesting or introduced species.
- Energy is lost through respiration, movement, waste and uneaten material, so chains are short and eating crops is more efficient than feeding crops to livestock.
SECTION 02
Ecological pyramids
Number pyramids count organisms and may be inverted. Biomass better represents living material but is a snapshot and may require destructive sampling.
DETAILED EXPLANATION
- Energy pyramids show transfer per area per time and are always upright.
- A trophic level is a feeding position.
SECTION 03
Carbon and nitrogen cycles
Matter cycles although energy flows one way. Photosynthesis removes carbon dioxide; respiration, decomposition and combustion return it. Feeding transfers carbon and fossil fuels store it.
DETAILED EXPLANATION
- Decomposers form ammonium; nitrifying microbes form nitrate.
- Nitrogen fixation by bacteria or lightning adds usable nitrogen; plants absorb nitrate to make proteins.
- Feeding transfers protein; deamination returns nitrogen compounds; denitrifying microbes release nitrogen gas.
SECTION 04
Populations
A population is one species in one area at one time; a community contains all populations; an ecosystem includes community and physical environment.
DETAILED EXPLANATION
- Growth depends on food, competition, predation and disease.
- Sigmoid phases are lag, exponential, stationary and death.
- At carrying capacity, limiting factors make births and deaths approximately balance.
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Energy enters as light and leaves as heat.
- Food webs connect feeding.
- Energy pyramids show transfer best.
- Nutrients cycle.
- Limiting factors shape populations.